Various cardiovascular surgical procedures, including repair or replacement of aortic, mitral and other heart valves, repair of congenital defects, coronary artery bypass grafting, and treatment of aneurysms, involve arrest of cardiac function. In such procedures, a cardiopulmonary bypass ("CPB") system must be used to oxygenate the blood and maintain circulation of the oxygenated blood through the patient during the entire time the heart is arrested. Typically, the components of the CPB system include, in sequence, one or more cannulae which are inserted into one or more major veins, such as the inferior vena cava, or into the heart itself for draining or withdrawing deoxygenated blood from the patient, a reservoir for collecting the venous blood, an oxygenator for removing CO.sub.2 from and oxygenating the deoxygenated blood, a filtration unit, an arterial pump for pumping the oxygenated blood back into the patient, and a cannulae which is inserted into a major artery such as the aorta or femoral artery for delivering oxygenated blood to the patient. The components of the CPB system additionally include tubing for carrying the blood throughout the CPB system.
Before the CPB system is connected to the patient, a sterile balanced electrolyte solution is added to the tubing and the reservoir to prime the system and thus prevent the introduction of air into the vascular system of the patient. After the CPB system is connected to the patient, the blood and the electrolyte solution, which are together defined herein as the "perfusate," are pumped into the patient through the arterial cannulae at a flow rate of approximately 0.0 to 7.0 (L/min) to ensure adequate perfusion of the patient's organs during surgery. Otherwise, it is necessary to cool the patient. It is also necessary to remove blood from the patient through the venous cannulae at a similar flow rate, which is comparable to the arterial flow rate to prevent distention of the patient's vascular system. Matching the venous flow rate to the arterial flow rate also helps to maintain the volume of the perfusate.
A number of techniques have also been developed to augment venous drainage and ensure an adequate venous flow rate. The most commonly used technique employs gravity to siphon venous blood from the body. Typically, tubing having a diameter of approximately 1/2" or 3/8" is used for the venous cannulae and the blood reservoir is placed on or near the floor to maximize the siphon effect. The oxygenator and arterial pump are also placed on or near the floor to minimize the length of tubing used to connect these CPB components to the reservoir. However, a considerable length of tubing, typically about 40 to 80 inches, is still needed to connect the patient to the reservoir and to the arterial pump. Thus, the patient's blood interacts with a large surface area of tubing which increases the possibility of hemodilution, where the tubing is primed, as well as hemolysis.
Moreover, since the priming volume of the electrolyte solution which is provided to the patient is dependent on the length and diameter of the tubing used, the gravity assisted technique requires a large priming volume. In a conventional cardiopulmonary bypass system, over 40% of the priming volume may be used to fill such tubing, which may be as high as 2400 ml of priming volume. High priming volumes may dilute the patient's red blood cells, platelets, and plasma proteins. Such hemodilution may lead to complications during recovery. For example, patients having diluted platelets and plasma proteins may be more likely to suffer from bleeding problems due to inadequate coagulation. Such patients may be more likely to suffer from post-operative anemia and thus require transfusions.
The large diameter venous cannulae that are required for the gravity assisted technique also have the potential to cause difficulties for surgeons using recently developed minimally invasive procedures for the cardiovascular surgery. In such procedures, a small incision, typically 10 cm, is made to gain access to the heart. If venous cannulae having a large diameter are inserted through this incision, the remaining working space available to the surgeon is significantly reduced.
Another technique for augmenting venous drainage uses a centrifugal pump connected to the venous line as described in L. Solomon et al., Augmented Femoral Venous Return, Ann. Thorac. Surg. (1993) 55:1262-3. Typically, in this technique, a venous cannulae is inserted into the right femoral vein and then guided to and positioned in the right atrium. The arterial portion of the vascular system is accessed through a cannulae which is inserted into the left femoral artery. Unfortunately, such femoral-femoral cannulation can lead to complications such as thrombophlebitis, wound infections, and dissections.
Moreover, the inclusion of additional centrifugal pumps to the CPB system has disadvantages. For example, such addition increases the amount of tubing required to carry the blood through the system, and thus increases the priming volume of electrolyte solution and contributes to greater hemodilution. Adding centrifugal pumps and increasing tubing used by the CPB system also increases the potential for hemolysis. Finally, including additional pumps also adds to the overall cost of the CPB system.
Thus, it is desirable to have a new CPB system which overcomes the disadvantages of the currently used systems. A CPB system which reduces the required priming volume of electrolyte solution would be especially desirable for cardiovascular surgeries conducted on both adult and pediatric patients. A CPB system which employs venous cannulae having a diameter smaller than those cannulae currently used in the gravity assisted venous drainage technique would also be especially desirable for minimally invasive procedures. A CPB system which eliminates the necessity of positioning the oxygenator and arterial pump a significant distance below the patient would also be desirable, as it would reduce the tubing length and prime. The CPB system which does not require additional, expensive equipment, such as a centrifugal pump, would also be advantageous.